In the past many attempts have been made to produce thin films of semiconductor materials by epitaxially depositing a layer of semiconductor material on a substrate and subsequently removing the film. Deposition techniques include physical vapor deposition (PVD) by evaporation of an elemental semiconductor material such as germanium, and chemical vapor deposition (CVD) by the decomposition of a precursor such as germane. Much of the demand for semiconductor thin films is for high quality single crystal films and the production of thin single crystal film requires single crystal substrates. Sodium chloride has been described in the prior art as being suited for the production of thin semiconductor films. Sodium chloride has a similar crystal structure and lattice parameter to several semiconductor materials including germanium, silicon and gallium arsenide.
It has been proposed to produce thin films of semiconductors on sodium chloride using both the previously mentioned PVD and CVD techniques. However, attempts to accomplish this have not been entirely successful due to the interaction of the sodium chloride substrate with the applied semiconductor layer. Both the PVD and CVD processes require that the substrate be heated to encourage the formation of an adherent single crystal (epitaxial) coating. At the temperature required the sodium chloride substrate sublimes and the sublimed material contaminates the semiconductor film and suppresses epitaxial growth. Recently, a plasma assisted deposition technique has been developed and is a particularly preferred process. This process permits the substrate to be maintained at a relatively low temperature while the precursor gas is dissociated in a plasma, thus the energy of the plasma is substituted for a portion of the thermal energy employed by the prior art for the decomposition process. This process will be briefly described below. It is more fully described and claimed in copending application U.S. Ser. No. 266,545 and now U.S. Pat. No. 4,421,592 Dec. 20, l983, titled, Plasma Assisted Deposits Of Semiconductor Films on Salt Substrates filed on even date herewith and sharing a common inventorship with the present case. In the process the reaction chamber is operated at a pressure of about 1 torr. A plasma zone is produced either through inductive coupling or capacitive coupling of RF energy to the precursor gas which is introduced into the plasma zone and flows through the plasma and over the substrate which is located within the plasma zone. Through this technique the substrate may be maintained at a temperature of about 450.degree. C. rather than the 600.degree. to 700.degree. C. temperature required by the prior art processes which relied entirely upon the substrate thermal energy to decompose the precursor gas. At this lower temperature it is a surprising result that high deposition rates can be obtained. While reference has been made to the use of sodium chloride as a substrate, it is clear that the other alkali halide salts may be similarly employed and in fact mixtures of these salts may be employed to produce a crystal substrate having a precisely controlled lattice parameter.
The prior art has taken two approaches to substrate fabrication. Most commonly, the prior art has used a single crystal semiconductor substrate which is coated with a thin intermediate layer of a material such as sodium chloride and this intermediate layer forms the actual substrate surface upon which the desired semiconductor thin film is applied. This is described, for example, in U.S. Pat. Nos. 4,116,751 and 3,993,533. The second approach taken by the prior art is to use a bulk single crystal of sodium chloride as a substrate. This is shown for example in U.S. Pat. Nos. 3,158,511 and 3,186,880.
The difficulty with using sodium chloride as a substrate is that the sodium chloride has a substantially greater coefficient of thermal expansion than do the semiconductor materials of interest. Thus, even though deposition may be satisfactorily achieved at a temperature of 500.degree. to 800.degree. C., upon cooling to room temperature the sodium chloride substrate contracts substantially more than the deposited semiconductor film and this can lead to buckling or other physical damage to the semiconductor layer. This problem has not been to our knowledge successfully solved by the prior art.
The prior art has suggested removal of the sodium chloride substrate or sodium chloride interlayer material through the use of sublimation (U.S. Pat. No. 3,186,880), dissolution of the substrate as for example by water (U.S. Pat. No. 3,158,511), and in the case of the sodium chloride intermediate layer, it has been proposed to heat the substrate, intermediate layer and semiconductor layer to a temperature where the intermediate layer was molten and then to physically peel or slide off the semiconductor surface layer (U.S. Pat. No. 4,116,751). None of these removal approaches have been entirely successful. Thus it is the purpose of the present invention to particularly describe a method for the removal of semiconductor thin films from bulk alkali halide salt substrates. This removal process when used in combination with the deposition process described in co-pending application, Plasma Assisted Deposits Of Semiconductor Films On Salt Substrates, filed on even date herewith provides a complete fabrication technique for the low cost production of high quality single crystal semiconductor films.